Hydraulic fracturing is a well stimulation technique that uses pressurized liquid to fracture rock. A fracking fluid is injected at high pressure into a wellbore to create and stimulate fractures within rock formations to promote the production of hydrocarbons from the well.
It is common when carrying out hydraulic fracturing to use drop balls, often referred to as frac balls, to isolate multiple different zones for stimulation within a formation. A series of packers is inserted into the wellbore at spaced apart intervals for isolating one zone from an adjacent zone. A drop ball having a predetermined diameter is dropped through the wellbore to selectively engage one packer in order to prevent fluid flow through that packer. The zone above that packer is then isolated, and that isolated zone can be treated or stimulated by the injection of fracturing fluid, which enters the formation through perforations in the casing.
Subsequently, a drop ball having a second predetermined diameter is dropped to block a different packer, typically that is located uphole of the previously blocked packer, to isolate a different zone uphole of the second packer for stimulation. This process is repeated until all desired zones have been stimulated. It is noted that in horizontally drilled wells, a first zone that is uphole of an adjacent zone may be positioned horizontally rather than vertically adjacent to the first zone.
Typically, the packers are arranged within the wellbore so that the most downhole packer will be blocked by the drop ball having the smallest diameter, and the most uphole packer will be blocked by the drop ball having the largest diameter. The drop balls are thus generally introduced in order from the drop ball having the smallest diameter, through drop balls having successively increasing diameters, to the drop ball having the largest diameter. In some cases, the diameter of the drop balls increases by approximately 1/16 inch per successively dropped ball. In some cases, the diameter of the drop balls produced by some manufacturers has a tolerance of +0.000, −0.003 inches.
The order in which the drop balls are introduced into the wellbore is very important, as dropping a ball having an incorrect diameter into the wellbore (i.e. dropping the drop balls out of order of the intended sequence of drop ball diameters) interferes with the hydraulic fracturing process. The result of dropping a ball having an incorrect diameter into the wellbore may be that certain zones are not stimulated by hydraulic fracturing. This can result in potentially significant economic losses, as hydrocarbons that would have been recovered had the hydraulic fracturing been carried out correctly are not recovered.
Documents of potential interest with respect to the technology described in this specification include:                U.S. Pat. No. 6,302,199 to Hawkins et al.;        U.S. Pat. No. 8,636,055 to Young et al.;        U.S. Pat. No. 9,109,422 to Ferguson et al.;        U.S. Pat. No. 9,291,024 to Artherholt et al.;        U.S. Pat. No. 9,291,025 to McGuire;        U.S. Pat. No. 9,739,111 to Beason et al.;        U.S. Pat. No. 9,447,652 to Artherholt et al.;        US 2008/0223587 to Cherewyk; and        US 2017/0022777 to Allen et al.        
There is a need for improved mechanisms and methods for ensuring that drop balls have the desired diameter before being introduced into a wellbore.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.